Bright nickel plating baths are used in the automotive, electrical, appliance, hardware and other industries. The most important functions of bright nickel plating are as an undercoating for chromium plating, helping finishers achieve a smooth bright finish and providing a significant amount of corrosion protection.
For decorative plated parts that need a high level of basis metal corrosion protection, semi-bright nickel deposits are almost always used in conjunction with subsequent deposits of bright nickel and chromium. The semi-bright nickel deposit is typically between about 60 and 70 percent of the total nickel deposited on the part, which offers the highest level of basis metal corrosion protection with the lowest total nickel thickness and the best appearance.
The most common nickel plating bath is a sulfate bath known as a Watts bath. In addition, in order to achieve bright and lustrous appearance of the nickel plating deposit, organic and inorganic agents (brighteners) are often added to the electrolyte. The types of added brighteners and their concentrations determine the appearance of the nickel deposit, i.e., brilliant, bright, semi-bright, satin, etc.
Traditionally, coumarin has been used to obtain a high-leveling, ductile, semi-bright and sulfur-free nickel deposit from a Watts nickel bath. However, coumarin-free solutions are now available. A semi-bright nickel finish is semi-lustrous, as the name implies, but it was specifically developed for its ease of polishing and buffing. In the alternative, if subsequently bright nickel is plated, buffing can be eliminated. Brightness and smoothness are dependent on operating conditions.
One of the reasons that semi-bright nickel finishes are so easily buffed and/or polished is that the structure of the deposit is columnar, whereas the structure of a bright nickel finish is plate-like (lamellar). However, the structure of the deposit can be changed with various additives, a change in pH, current density or an increase in solution agitation, which is not a problem unless it affects properties of the deposit such as internal stress.
Internal stress of the plated nickel deposit can be compressive or tensile. Compressive stress is where the deposit expands to relieve the stress. In contrast, tensile stress is where the deposit contracts. Highly compressed deposits can result in blisters, warping or cause the deposit to separate from the substrate, while deposits with high tensile stress can also cause warping in addition to cracking and reduction in fatigue strength.
The use of coumarin as an additive in nickel electroplating baths, especially semi-bright nickel processes, to produce ductile, lustrous deposits with excellent leveling is well known. High concentrations of coumarin in the bath gives the best leveling results on one side, but such high coumarin concentrations also result on the other side in a high rate off formation of detrimental breakdown or degradation products. These degradation products are objectionable in that they can cause uneven, dull gray areas that are not easily brightened by a subsequent bright nickel deposit, they can reduce the leveling obtained from a given concentration of coumarin in the plating bath, and they can reduce the beneficial physical properties of the nickel deposits.
The use of various additives, such as formaldehyde and chloral hydrate has also been suggested to help overcome the undesirable effects of the coumarin degradation products. However, the use of such additives has certain limitations because even moderate concentrations of these materials not only increase the tensile stress of the nickel electrodeposits, but also significantly reduces the leveling action of the coumarin.
Even when since decades plating suppliers have proposed many bath formulations which claim to level as well as a coumarin bath, up to now, none of these baths formulations have met all of the necessary criteria.
As explained above, while the leveling of coumarin is exceptional coumarin has a disagreeable odor, breaks down and forms harmful degradation products, and these degradation products can only be removed by batch carbon treatments of the plating bath. These treatments are expensive and time consuming and normally must be done at least monthly and in some cases, even weekly.
DE 196 10 361 A1 discloses a process for a galvanic deposition of semi-bright nickel coatings on a substrate, wherein said substrate has been treated by an acidic aqueous galvanic bath comprising a cyclic N-allyl- or N-vinyl-ammonium compound, in particular based on pyridinium, as brightener additive.
However, none of the known prior art suggests a way to achieve the desired complex combination of good deposit properties of a semi-bright nickel or nickel alloy coating having good glance properties without generating high internal stress values. Prior art baths have solely been successful to achieve semi-bright nickel or nickel alloy coatings exhibiting some good properties while other properties have kept bad or turned bad, such a s combinations of good glance and high internal stress; or of bad glance and low internal stress.